Metal-coated steel strip

ABSTRACT

A method of forming an Al—Zn—Si—Mg alloy coating on a strip that includes dipping strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip, with the Al—Zn—Si—Mg alloy containing in % by weight: Al: 2 to 19%, Si: 0.1 to 2%, Mg: 1 to 10%, and Zn: 80 to 97%, and with the bath having a molten metal layer and a top dross layer on the metal layer, and the method including providing Ca in the composition of the bath to minimise the top dross layer in the molten bath.

BACKGROUND OF THE INVENTION

The present invention relates to the production of strip, typicallysteel strip, which has a corrosion-resistant metal alloy coating thatcontains aluminium-zinc-silicon-magnesium as the main elements in thealloy in the following ranges in % by weight:

Al: 2 to 19%

Si: 0.1 to 2%

Mg: 1 to 10%

Zn: 80 to 97%

The above alloy is described and claimed in Australian patent 758643entitled “Plated steel product, plated steel sheet and precoated steelsheet having excellent resistance to corrosion” in the name of NipponSteel Corporation.

The above alloy is hereinafter referred to as an “Al—Zn—Si—Mg alloy”.

The Al—Zn—Si—Mg alloy coating may contain other elements that arepresent as deliberate alloying additions or as unavoidable impurities.Hence, the phrase “Al—Zn—Si—Mg alloy” is understood herein to coveralloys that contain such other elements as deliberate alloying additionsor as unavoidable impurities. The other elements may include by way ofexample any one or more of Fe, Ti, Cu, Ni, Co, Ca, Mn, Be, Sr, Ca, Cr,and V.

SUMMARY OF THE INVENTION

In particular, the present invention relates to a hot-dip metal coatingmethod of forming an Al—Zn—Si—Mg alloy coating on a strip that includesdipping uncoated strip into a bath of molten Al—Zn—Si—Mg alloy andforming a coating of the alloy on the strip.

The present invention is concerned with minimising the amount of topdross in the alloy coating bath. Top dross is undesirable from theviewpoints of cost of production and coating quality, as is discussedfurther below.

The term “top dross” is herein understood to include any one or more ofthe following components on or near the surface of the molten bath:

-   -   (a) an oxide film on the surface of a molten bath,    -   (b) molten metal droplets covered by an oxide film,    -   (c) gas bubbles having an oxide film as the wall of the bubbles,    -   (d) intermetallic particles that are formed in the coating bath,        including particles covered by an oxide film, and    -   (e) combinations of any two or more of gas, molten metal, and        intermetallic particles covered by an oxide film.

Items (b), (c), (d), and (e) can be described as the result ofentrainment of molten metal, gas, and intermetallic particles in theoxide film on or near the surface of the molten bath.

International application PCT/AU2011/000069 entitled “Metal-Coated SteelStrip” in the name of the applicant is concerned with minimising theamount of top dross in an alloy coating bath of an alloy that containsaluminium-zinc-silicon-magnesium as the main elements in the alloy. Theinvention described and claimed in the International application isbased on laboratory work and line trials on coating bath alloycompositions containing, 53% Al, 43% Zn, 2% Mg, 1.5% Si, and 0.5% Fe,with the percentages being percentages by weight, and different amountsof Ca and Sr in the baths carried out by the applicant. The coatingalloys were coated onto steel strip. The invention was made during thecourse of a research and development project that investigated theaddition of Mg to a known corrosion resistant metal coating composition,namely 55% Al—Zn—Si, that is used widely in Australia and elsewhere forbuilding products, particularly profiled wall and roofing sheets. At thetime the invention of the International application was made, theaddition of Mg to this known composition of 55% Al—Zn—Si coatingcomposition had been proposed in the patent literature for a number ofyears, see for example US patent 6,635,359 in the name of Nippon SteelCorporation, but Al—Zn—Si—Mg coatings on steel strip were notcommercially available in Australia. More particularly, it had beenestablished that when Mg is included in a 55% Al—Zn coating composition,Mg brings about certain beneficial effects on product performance, suchas improved cut-edge protection. However, the applicant found in theresearch and development project that Mg-containing molten 55% Al—Zncoating metal is susceptible to increased levels of top dross generationcompared to molten 55% Al—Zn coating metal that does not contain Mg.During a line trial involving hot-dip metal coating a Mg-containing 55%Al—Zn alloy onto a steel strip conducted by the applicant it was shownthat the level of top dross generated in the coating bath was 6 to 8times that of the top dross formed in a 55% Al—Zn alloy coating bathwithout Mg addition. This amount of top dross generated has asignificant impact on the cost of production of Mg-containing 55% Al—Znalloy coated steel and product quality. The applicant found in theresearch and development project that the amount of top dross could begreatly reduced by the addition of Ca and/or Sr to a coating bath.

The applicant has now found that there is substantial top drossgenerated in hot dip coating steel strip in a molten bath containing theabove-described Al—Zn—Si—Mg alloy containing in % by weight: Al: 2 to19% , Si: 0.1 to 2% , Mg: 1 to 10% , and Zn: 80 to 97% and that the topdross is undesirable in terms of cost of production and product quality.The applicant had not anticipated that top dross would have been assignificant an issue with the above-described Al—Zn—Si—Mg alloy.

The above discussion is not to be taken as an admission of the commongeneral knowledge in Australia and elsewhere.

The applicant has been able to reduce the top dross levels in moltenAl—Zn—Si—Mg alloy baths containing in % by weight: Al: 2 to 19% , Si:0.1 to 2% , Mg: 1 to 10% , and Zn: 80 to 97% by the addition to moltenbaths of Ca, and the reduction in top dross levels has lead to benefitsin terms of production costs and product quality.

According to the present invention there is provided a method of formingan Al—Zn—Si—Mg alloy coating on a strip that includes dipping strip intoa bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy onthe strip, with the Al—Zn—Si—Mg alloy containing in % by weight: Al: 2to 19% , Si: 0.1 to 2% , Mg: 1 to 10% , and Zn: 80 to 97% , and with thebath having a molten metal layer and a top dross layer on the metallayer, and the method including providing Ca in the composition of thebath to minimise the top dross layer in the molten bath.

The Al—Zn—Si—Mg alloy may contain other elements that are present asdeliberate alloying additions or as unavoidable impurities. Hence, thephrase “Al—Zn—Si—Mg alloy” is understood herein to cover alloys thatcontain such other elements as deliberate alloying additions or asunavoidable impurities. The other elements may include by way of exampleany one or more of Fe, Ti, Cu, Ni, Co, Ca, Mn, Be, Sr, Cr, and V.

The composition of the bath may include more than 50 ppm Ca. It is notedthat all references to ppm in the specification are references to ppm byweight.

The composition of the bath may include more than 100 ppm Ca.

The composition of the bath may include more than 200 ppm Ca.

The composition of the bath may include more than 250 ppm Ca.

The composition of the bath may include more than 300 ppm Ca.

The composition of the bath may include less than 2000 ppm Ca.

The composition of the bath may include less than 1500 ppm Ca.

The composition of the bath may include less than 1000 ppm Ca.

It is noted that the references to amounts of elements such as Ca aspart of the composition of a molten bath are understood herein to bereferences to the concentrations of the elements in the molten metallayer of the bath as opposed to the top dross layer in the bath. Thereason for this is that it is the standard practice of the applicant tomeasure bath concentrations in the molten metal layers of molten baths.

It is also noted that the applicant found that Ca tends to segregate tothe top dross layer of molten baths and, as a consequence the top drosslayer becomes enriched with respect to Ca when compared to the metallayer. Specifically, if there is “x” wt. % of Ca in the molten metallayer of a molten bath, there will be a higher concentration of theelement in the top dross layer of the bath. For example, the applicantfound in laboratory work that in a bath with a nominal bath compositionof 90 ppm Ca, the Ca content of the top dross layer increased to 100 ppmCa. Similarly, the applicant found that in a bath with a nominalcomposition of 400 ppm Ca, the top dross layer was enrichedsubstantially to 600 ppm. In practice, this means that, if it isrequired that there be “x” wt. % of Ca in the molten metal layer of amolten bath, it will be necessary to add an amount of Ca that is greaterthan “x” wt. % in the total bath to compensate for the higherconcentration of Ca that will segregate to the top dross layer.

The Ca may be added to the bath as required. It could be by way ofspecific additions of Ca compounds on a continuous or a periodic basis.It could also be by way of the inclusion of Ca in Al and/or Zn ingotsthat are provided as feed materials for the bath.

The method may include controlling the concentration of Ca in the bathto minimise the top dross layer in the molten bath.

The method may include controlling the composition of the bath tominimise the top dross layer in the bath by periodically monitoring theconcentration of Ca that is in the bath, and adding Ca as required tomaintain the bath composition for the element.

In a situation in which the Ca is part of ingots of other elements thatare in the composition in the bath, the method may include selecting anyone or more of the sizes of the ingots, the timing of the addition ofthe ingots, and the sequence of the addition of the ingots to maintainthe concentration of Ca substantially constant or within a preferredrange of +or −10% for the elements.

The Al—Zn—Si—Mg alloy may include more than 8% by weight Al.

The Al—Zn—Si—Mg alloy may include more than 10% by weight Al.

The Al—Zn—Si—Mg alloy may include less than 15% by weight Al.

The Al—Zn—Si—Mg alloy may include less than 12% by weight Al.

The Al—Zn—Si—Mg alloy may include more than 0.3% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 1% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 2% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 2.5% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 3% by weight Mg.

The Al—Zn—Si—Mg alloy may include less than 5% by weight Mg.

The Al—Zn—Si—Mg alloy may include less than 4% by weight Mg.

The Al—Zn—Si—Mg alloy may include more than 0.15% by weight Si.

The Al—Zn—Si—Mg alloy may include less than 1.2% by weight Si.

The Al—Zn—Si—Mg alloy may include less than 1% by weight Si.

The Al—Zn—Si—Mg alloy may include less than 0.25% by weight Si.

According to the present invention there is also provided an Al—Zn—Si—Mgalloy coating on a strip produced by the above-described method.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described further by way of example withreference to the accompanying drawings of which:

FIG. 1 is a schematic drawing of one embodiment of a continuousproduction line for producing steel strip coated with an Al—Zn—Si—Mgalloy in accordance with the method of the present invention; and

FIG. 2 is a graph of the mass of dross versus Ca concentration formolten Al—Zn—Si—Mg alloy baths with and without Ca in experiments ondross generation carried out by the applicant.

DETAILED DESCRIPTION

With reference to FIG. 1, in use, coils of cold rolled steel strip areuncoiled at an uncoiling station 1 and successive uncoiled lengths ofstrip are welded end to end by a welder 2 and form a continuous lengthof strip.

The strip is then passed successively through an accumulator 3, a stripcleaning section 4 and a furnace assembly 5. The furnace assembly 5includes a preheater, a preheat reducing furnace, and a reducingfurnace.

The strip is heat treated in the furnace assembly 5 by careful controlof process variables including:(i) the temperature profile in thefurnaces, (ii) the reducing gas concentration in the furnaces, (iii) thegas flow rate through the furnaces, and (iv) strip residence time in thefurnaces (i.e. line speed).

The process variables in the furnace assembly 5 are controlled so thatthere is removal of iron oxide residues from the surface of the stripand removal of residual oils and iron fines from the surface of thestrip.

The heat treated strip is then passed via an outlet snout downwardlyinto and through a molten bath containing an Al—Zn—Si—Mg alloy held in acoating pot 6 and is coated with Al—Zn—Si—Mg alloy. The Al—Zn—Si—Mgalloy is maintained molten in the coating pot by use of heatinginductors (not shown). Within the bath the strip passes around a sinkroll and is taken upwardly out of the bath. Both surfaces of the stripare coated with the Al—Zn—Si—Mg alloy as it passes through the bath.

After leaving the coating bath 6 the strip passes vertically through agas wiping station (not shown) at which its coated surfaces aresubjected to jets of wiping gas to control the thickness of the coating.

The coated strip is then passed through a cooling section 7 andsubjected to forced cooling.

The cooled, coated strip is then passed through a rolling section 8 thatconditions the surface of the coated strip.

The coated strip is thereafter coiled at a coiling station 10.

As is indicated above, the applicant has found that Al—Zn—Si—Mg alloycoating baths containing in % by weight: Al: 2 to 19% , Si: 0.1 to 2% ,Mg: 1 to 10% , and Zn: 80 to 97% generate substantial amounts of topdross in the baths that is undesirable in terms of production costs andproduct quality.

As discussed above, the applicant conducted a number of laboratoryexperiments to determine whether it is possible to reduce the amount ofdross generated in Al—Zn—Si—Mg alloy baths having compositions, in % byweight, of: Al: 2 to 19% , Si: 0.1 to 2% , Mg: 1 to 10% , and Zn: 80 to97% .

As discussed above, the applicant found that it was possible tosignificantly reduce the level of top dross by the addition of Ca tosuch Al—Zn—Si—Mg alloys in coating baths.

The experimental results for experiments of 3 hours duration on theeffect of Ca additions to coating baths on the level of top drossgeneration in Al—Zn—Si—Mg alloy coating baths is summarized in FIG. 2.

The experimental work was carried out on the following alloycompositions, in wt. % for (a) an Al—Zn—Si—Mg alloy and (b) this alloyplus parts per million (ppm) Ca additions to the composition:

-   -   Alloy: Al: 11.2% Al; Mg: 3% ; Si: 0.19% ; Zn: balance; and        unavoidable impurities    -   Alloy+500 ppm (0.05 wt. % ) Ca.    -   Alloy+750 ppm (0.075 wt. % ) Ca.    -   Alloy+1500 ppm (0.15 wt. % ) Ca.

It is noted that the concentrations of Ca are the concentrations ofthese elements in the metallic parts of molten baths.

In the experimental work the top dross generation was simulated using alaboratory melting furnace and an overhead mechanical stirrer. Thelaboratory set-up consisted of the following components:

-   -   A melting furnace with clay graphite crucible.    -   A variable speed overhead mechanical stirrer with a support        stand.    -   Dross collector cup machined from high density sintered        boron-nitride ceramic and having a series of drainage holes in        the bottom of the cup and a series of upstanding handles to        allow the cup to be positioned and removed from the crucible.    -   Stainless steel impellor shaft.    -   Impellor machined from high density sintered boron nitride        ceramic.

The dross collector cup and the impellor were fabricated from a hightemperature material that is non-wetting to the coating alloy tested inthe experimental work. The sintered boron nitride ceramic of thesecomponents provided excellent non-wetting characteristics and hightemperature stability in the coating bath.

For each experiment, 15 kg of the coating alloy of a requiredcomposition was formed in the crucible and held at the processtemperature of 460° C. The dross collector cup was then inserted intothe molten bath and was retained in the bath until the melt temperaturereached the process temperature. Then the shaft impellor assembly waslowered into the bath until the impellor just touched the surface of themelt. The stirrer motor was then switched on and the stirring speed wasadjusted to 60 RPM. This experimental set-up resulted in shearing of thesurface of the bath without creating a vortex so that at each revolutionof the impellor a fresh melt was continuously exposed to air to generatedross. The dross generated was pushed to the side of the crucible andaccumulated on the side of the crucible. At the end of each experimentthe accumulated dross was removed from the crucible by lifting the drosscollector cup from the crucible and allowing excess entrained bath metalto drain into the crucible via holes in the dross collector cup. Whatwas left in the dross collector cup comprised the entrained bath metaland dross intermetallic particles covered with oxide film. This retainedmaterial was the top dross generated in each experiment.

The experiments were conducted for durations of 0.5, 1, 2, and 3 hrs.

After each experiment the dross collected was removed and weighed andthe results are plotted for the 3 hour experiments as shown in FIG. 2.

FIG. 2 is a graph of the mass of dross generated versus Ca concentrationfor the molten alloy baths.

FIG. 2 clearly shows that the level of top dross generated in anAl—Zn—Si—Mg alloy bath can be significantly reduced by additions of Cato coating baths. More particularly, FIG. 2 shows that the amount of topdross decreases significantly with increasing amounts of Ca in thecoating baths.

In practice, the Ca may be added to a coating bath as required. It couldbe by way of specific additions of Ca compounds on a continuous or aperiodic basis. It could also be by way of the inclusion of Ca and/or inAl and/or Zn ingots that are provided as feed materials for the bath.

Many modifications may be made to the present invention described abovewithout departing from the spirit and scope of the invention.

1. A method of forming an Al—Zn—Si—Mg alloy coating on a strip thatincludes dipping the strip into a bath of molten Al—Zn—Si—Mg alloy andforming a coating of the alloy on the strip, with the Al—Zn—Si—Mg alloycontaining in % by weight: Al: 2 to 19% , Si: 0.1 to 2% , Mg: 1 to 10% ,and Zn: 80 to 97% , and with the bath having a molten metal layer and atop dross layer on the metal layer, and the method including providingCa in the composition of the bath to minimise the top dross layer in themolten bath.
 2. The method defined in claim 1 wherein the Al—Zn—Si—Mgalloy contains other elements that are present as deliberate alloyingadditions or as unavoidable impurities.
 3. The method defined in claim 1wherein the composition of the bath includes more than 50 ppm Ca.
 4. Themethod defined in claim 1 wherein the composition of the bath includesmore than 100 ppm Ca.
 5. The method defined in claim 1 wherein thecomposition of the bath includes more than 200 ppm Ca.
 6. The methoddefined in claim 1 wherein the composition of the bath includes lessthan 2000 ppm Ca.
 7. The method defined in claim 1 includes adding Ca byway of specific additions of Ca compounds on a continuous or a periodicbasis.
 8. The method defined in claim 1 includes adding Ca in Al and/orZn ingots that are provided as feed materials for the bath.
 9. Themethod defined in claim 1 and further including controlling thecomposition of the bath to minimise the top dross layer in the bath byperiodically monitoring the concentration of Ca that is in the bath andadding Ca as required to maintain the bath composition for the element.10. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includesmore than 8% by weight Al.
 11. The method defined in claim 1 wherein theAl—Zn—Si—Mg alloy includes less than 15% by weight Al.
 12. The methoddefined in claim 1 wherein the Al—Zn—Si—Mg alloy includes more than 0.3%by weight Mg.
 13. The method defined in claim 1 wherein the Al—Zn—Si—Mgalloy includes more than 2% by weight Mg.
 14. The method defined inclaim 1 wherein the Al—Zn—Si—Mg alloy includes less than 5% by weightMg.
 15. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloyincludes more than 0.15% by weight Si.